Standalone px4 stable (#22467)

simulation gazebo: move the gazebo models to submodule, allow for operation with external gazebo instance, independent of startup order. Allows drag an drop of models from gazebo fuel.

* rolled back updates

Signed-off-by: frederik <frederik@auterion.com>

* fixing empy

Signed-off-by: frederik <frederik@auterion.com>

* Update GZBridge.cpp to lower drop position 

Dropping from 1m leads to movement in the rc_cessna. Dropping from 0.5m leads to no movement.

* Update STANDALONE env variable.

* Update STANDALONE env_variable on GZBridge

* Update src/modules/simulation/gz_bridge/GZBridge.cpp

Co-authored-by: Daniel Agar <daniel@agar.ca>

* test removal of x500

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* removed all models and reworked logic

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* remove model path in set_sdf_filename

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* filter resource path for world sdf

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* updated structure to keep old make px4_sitl

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* remove gz tools

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* import gz as submodule and reverse rc simulator logic

Signed-off-by: frederik <frederik.anilmarkus@gmail.com>

* [gz-sim]: source GZ_SIM_RESOURCE_PATH only if PX4 starts gz server

Signed-off-by: Beniamino Pozzan <beniamino.pozzan@gmail.com>

* Typo fix

---------

Signed-off-by: frederik <frederik@auterion.com>
Signed-off-by: frederik <frederik.anilmarkus@gmail.com>
Signed-off-by: Beniamino Pozzan <beniamino.pozzan@gmail.com>
Co-authored-by: Daniel Agar <daniel@agar.ca>
Co-authored-by: Beniamino Pozzan <beniamino.pozzan@gmail.com>

.gitmodules

@@ -76,3 +76,6 @@
 	path = src/lib/heatshrink/heatshrink
 	url = https://github.com/PX4/heatshrink.git
 	branch = px4
+[submodule "Tools/simulation/gz"]
+	path = Tools/simulation/gz
+	url = https://github.com/PX4/PX4-gazebo-models.git